Measuring devices are frequently used in automation and process control technology to ascertain a process variable, e.g. flow, fill level, pressure and temperature or some other physical and/or chemical process variable in a process flow. The assignee is active in the field of industrial automation and process control technology and produces industrial measuring devices and field devices. It sells these field devices, for example, among other things for fill level determination of a medium in a container, under the marks MICROPILOT, LEVELFLEX or DELTAPILOT.
One known measuring method among a large number of measuring methods for ascertaining the fill level in a container is the travel time measuring method. In the case of this measuring method, microwaves, for example, are transmitted via an antenna apparatus, and the echo waves reflected off the surface of the medium are detected, wherein the travel time of the measurement signal is a measure for the traveled distance. From half of the travel time, the fill level of the medium in a container can accordingly be ascertained. The echo curve represents, in such case, the total signal route as a function of time, wherein each measured value of the echo curve corresponds to the amplitude of an echo signal reflected off a surface at a particular distance. The travel time measuring method is essentially divided into two methods of ascertainment: In the case of time difference measurement, the time is ascertained, which a broadband wave signal pulse requires for a traveled path. In the case of frequency difference measurement (FMCW—Frequency-Modulated Continuous Wave), the emitted, frequency-modulated high-frequency signal is compared with the reflected, received, frequency-modulated high-frequency signal. In the following, no restriction is made to a particular method of ascertainment.
In the case of certain process applications, the measuring devices or their sensor elements are exposed to extreme conditions, e.g. high temperatures, high pressures and/or chemically aggressive materials. Especially microwave fill level measuring devices have temperature sensitive and/or pressure sensitive components, such as, for example, a measuring device electronics and transmitting and/or receiver elements for the microwaves.
In the high temperature range, predominantly aluminum oxide ceramic and/or synthetic materials resistant to high temperatures of up to 250° C., e.g. polyether ketones (PEEK), are used as temperature resistant material for process separation in the sensor element.
The process sealing of these process separation elements in the sensor element is, in the case of ceramics, most often implemented via soldering and/or graphite packing glands. In the case of use of synthetic materials as process separation elements, high temperature resistant, fluorine elastomers, such as, for example, perfluoro rubber (FFKM or FFPM), which are sold under the mark, Kalrez, are applied as sealing elements very near to the hot process.
In order to protect the measuring electronics in the measurement transmitter against high temperatures and pressures as well as against aggressive chemical materials, the sensor elements are hermetically sealed on the process side with sealing elements. By inserting a hermetically sealed sealing element into the hollow conductor of a horn antenna, a largest possible degree of safety is assured, since a second “safety element” seals the process in the case of a detachment of the modular, measurement active parts—e.g. an in-coupling unit/exciter element or the measuring device electronics—from the measurement passive parts, such as e.g. the antenna, for reasons of maintenance or repair.
In US 2005/0253751 A1, a modular construction of a horn antenna is described. The process separating element is embodied in the shape of a ceramic matching cone, which is brought into the hollow conductor, sealed by graphite packing rings.
Of disadvantage in the cited examples of embodiments of process sealing of the sensor element or of its process connection against the medium located in the process according to the methods known from the state of the art is:                that the soldering or glass melted closure of the process isolating element for sealing against the medium in the process is very complicated and expensive, and can only be applied in the case of ceramics;        that graphite packing glands do not have the required quality of sealing, especially at high temperatures, since the permeation of process medium through the graphite packing gland is no longer negligible at higher temperatures; and        that the elastomers are not resistant in the long term at temperatures >200° C., and with rising temperature, very rapidly display increasing aging phenomena and, as a result thereof, phenomena indicating lack of sealing.        